1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to an image forming apparatus incorporating a transfer bias optimizer that optimizes an electrical transfer bias used to transfer a toner image in electrophotographic imaging processes.
2. Discussion of the Background
In electrophotography, images developed with toner particles are transferred from one medium to another during several imaging processes. Many electrophotographic image forming apparatuses employ transfer devices to transfer toner images across a transfer nip or gap, in which a biasing member, such as a roller, brush, or corona electrode, provided with an electrical transfer bias, develops a transfer field that induces toner particles to move, or transfer, to correct locations on an intermediate belt or a sheet of paper.
A good image is obtained with a high density of toner and a high transfer rate at which toner transfers from one surface to another, which in turn is highly dependent on the transfer field developed with the transfer bias.
For example, with too low a bias voltage applied to the biasing member, the resulting transfer field is too weak to attract toner particles. On the other hand, too high a bias voltage makes too strong a transfer field that induces an electrical discharge in the transfer nip. In either case, an inappropriate transfer field reduces transfer rate and density of toner in resulting images. In particular, the electrical discharge in the transfer nip is known to disturb transfer of toner, and can cause “reverse transfer”, in which toner that has been transferred from an upstream photoconductor retransfers to a downstream photoconductor during sequential transfer of toner to a single receiving surface.
Thus, ensuring good image quality requires optimizing the transfer bias to obtain an appropriate transfer field. However, such optimization is difficult to achieve since the transfer bias is sensitive to variations in operating conditions, such as temperature and humidity, resistance of recording sheets, charge amounts of toner, and settings of specific print jobs.
Various techniques have been proposed to enhance transfer performance in image forming apparatuses.
Some conventional image forming apparatuses adjust conditions for printing according to actual transfer rates measured for toner images of a given test pattern. One such method forms a test image on a photoconductor drum, transfers it to a substrate, and adjusts print settings based on density of toner remaining on the photoconductor after transfer. Another method measures density of a test image on a photoconductor before and after transfer to an intermediate transfer belt, compares the measured densities, and adjusts electrical charges applied to the intermediate transfer belt based on the comparison results.
Designed to stabilize transfer rate at relatively low transfer bias voltages, these conventional methods do not account for effects of an excessive transfer bias on transfer performance, and therefore, cannot prevent image degradation due to an electric discharge occurring in the transfer gap. In this regard, several methods have been proposed for detecting occurrence of an electric discharge in a development nip defined between a photoconductor and a developer applicator during development of an electrostatic latent image.
For example, one conventional developing device detects an electrical discharge in a development nip by sensing a current flowing between electrodes submerged in developer upon voltage application, and adjusts agitation of the developer according to the detection results. Another conventional developing device measures density of a test image developed on a photoconductor to detect occurrence of an electrical discharge in a development nip, and adjusts a bias applied to move developer across the development nip.
Unfortunately, these detection techniques are designed for use in developing devices, and cannot be applied to the detection of an electrical discharge during transfer of developed toner images. Hence, what is needed is a transfer bias control system that can effectively optimize a transfer bias to provide a high transfer rate while still preventing electrical discharges in transferring toner images across a transfer gap.